This invention relates to an abnormal state diagnosis method for examining whether or not a gas sensor disposed in an exhaust passage of an internal combustion engine is in an abnormal state, and/or gas sensor control or diagnostic apparatus for such a gas sensor.
A three-way catalyst is widely used in internal combustion engines for motor vehicles and other applications, to purify CO, HC and NOx contained in exhaust gas mixture. Furthermore, in order to improve the efficiency of the purification, there is provided a feedback air-fuel ratio control system including a gas sensor disposed in the exhaust passage of the engine, for sensing the oxygen concentration in the exhaust gases, and a controller for controlling the mixture ratio or air-fuel ratio toward the theoretical air-fuel ratio by regulating a fuel supply quantity, such as a fuel injection quantity, to the engine in accordance with the sensed oxygen concentration. Recently, the feedback air-fuel ratio control system often employs, as the gas sensor, a wide-range or full-range air-fuel ratio sensor varying the sensor output linearly in accordance with the oxygen concentration in the exhaust gases, in order to improve the accuracy of the feedback control.
In a case in which the gas sensor is used for a long time period, there may be generated the time degradation such as a clogging of gas through holes formed in a protector of the gas sensor (protector for covering and protecting periphery of the sensing element), and porous portions for introducing the exhaust gas into the sensing element. By this degradation of the gas sensor, a response of a sensor output outputted in accordance with a concentration variation of a specified gas component in the exhaust gas becomes slow relative to a gas sensor (in a normal state) which is not deteriorated. Accordingly, this degradation may cause problems that the fuel economy is deteriorated, and that the exhaust gas is not sufficiently purified. Therefore, there are proposed methods of the deterioration diagnosis of the gas sensor by using sensor output value of the gas sensor.
For example, published Japanese Patent Applications, Publication Numbers H4-36651 and H8-177575 propose an air-fuel ratio control method and a self-diagnostic apparatus of an air-fuel ratio control system configured to diagnose whether or not the gas sensor is in the abnormal state by using a variation of the output of the gas sensor which corresponds to a start of an interruption of a fuel supply to an internal combustion engine, and a restart of the fuel supply after the fuel supply interruption.